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<br />community. Since 1950, an additional gaging station has been operated on the Rio Grande <br />conveyance channel; consequently, the flows shown in the table beginning in 1950 are the total <br />of the flows in both the river and the conveyance channel. Prior to July 1946, the stream gaging <br />station was operated by either the IBWC or the USBR. Since then, the gaging stations have <br />been operated by the USGS. <br /> <br />Figure 2-2 is a bar chart of the annual flows at San Marcial during 1896-1995, and has <br />been included in this report to show the variation in the annual inflow to the Project. The figure <br />also shows the 1o-year running average of these annual flows. A 10-year running average is <br />often useful in showing trends in hydrologic data. It can be seen in this table that the flows at <br />San Marcial tended to be relatively high during a period beginning in 1903 and extending to <br />1949, relatively low from about 1950 through 1981, and relatively high again since 1981. <br /> <br />Elephant Butte Reservoir has a maximum usable capacity of approximately 2,065,000 <br />acre-feet, as indicated by a 1988 survey, and is the primary storage facility for the Rio Grande <br />Project. It was constructed during 1912-16 with storage operation beginning in March 1915. It <br />is formed by Elephant Butte Dam, which is located 4 miles east of Truth or Consequences. <br />Elephant Butte Dam is a concrete gravity dam, 301 feet high and 1,674 feet long. The spillway <br />is an uncontrolled concrete ogee weir and concrete-lined chute on the right side of the dam with <br />four 10-foot diameter circular openings through the base of the weir. Each of the circular <br />openings is controlled by a circular gate. A spillway channel was added below the dam in 1921 <br />and modified in 1947. The outlets from the reservoir are located through the dam near its left <br />abutment. Four conduits serve as service outlets and are controlled by four 60-inch balance <br />valves. The two sluicing outlets are controlled by two 47-inch x 60-inch slide gates. There are <br />six penstock openings leading to the powerplant that were constructed in 1940. The nameplate <br />capacity of the powerplant is 24,000 kilowatts, made up by three generators, each having a <br />capacity of 8,100 kilowatts. At the spillway crest (elevation 4,407), the reservoir has a water <br />surface area of about 37,000 acres or about 58 square miles. The historical operation of <br />Elephant Butte Reservoir is considered in detail in Section 4 of this report. <br /> <br />In recent years, the USBR has reserved 50,000 acre-feet of the reservoir capacity to <br />provide flood protection for Truth or Consequences. Thus, the maximum usable capacity at the <br /> <br />2-5 <br /> <br />r - C. u':; o.~ <br />~,; \. <br />